The present disclosure relates to a zoom lens used in an interchangeable lens device of a lens-interchangeable digital camera and so forth, and an imaging device incorporating such a zoom lens.
Requirements for size reduction and portability improvement of the lens-interchangeable digital camera system are still strong. In particular, in recent years, these requirements are becoming stronger because of the rapid spread of the body of the lens-interchangeable digital camera having a mirrorless mechanism. In general, in the mirrorless lens-interchangeable digital camera, the flange focal distance is short and the body itself is also designed as small one compared with existing single-lens reflex cameras. Therefore, the lens is also required to have further reduced size so that the portability of the system may be not spoiled.
Furthermore, in recent years, a lens suitable for moving image photographing is required against the background of the spread of a lens-interchangeable digital camera having the moving image photographing function. In general, in the lens-interchangeable digital camera, a large-size imaging element is used and the actual focal length of the lens is long. Therefore, highly accurate focusing that allows withstanding against the shallow depth of field is necessary. In addition, it is also necessary for the camera to have a high-speed focusing function for object tracking. To achieve them, attainment of proper focusing sensitivity in the range from the wide angle end to the telephoto end and weight reduction of the focus lens group are required.
As the zoom lens used as the interchangeable lens of the lens-interchangeable camera, a multi-group zoom lens in which not less than three lens groups are moved is mainstream in recent years. In general, the multi-group zoom lens is advantageous in terms of aberration correction because it has high flexibility of change in the relative positions of the respective lens groups in association with magnification variation, and easily allows performance enhancement. Furthermore, by dispersing the magnification variation action to the respective lens groups, achievement of high magnification and reduction in the amount of movement of the respective groups in magnification variation are facilitated.
Japanese Patent Laid-open No. Hei 9-230238 (hereinafter, Patent Document 1) proposes a zoom lens in which a first lens group having a positive refractive index, a second lens group having negative refractive power, a third lens group having a positive refractive index, a fourth lens group having a positive refractive index, and a fifth lens group having a negative refractive index are deposed in that order from the object side. In this zoom lens, in magnification variation from the wide angle end to the telephoto end, at least the first lens group and the fifth lens group move toward the object side in such a manner that the air gap between the first lens group and the second lens group increases, and the air gap between the second lens group and the third lens group decreases, and the air gap between the fourth lens group and the fifth lens group decreases. Furthermore, at least the fourth lens group moves in such a manner that the on-axis distance between the second lens group and the fourth lens group decreases. In addition, the fourth lens group is moved along the optical axis to perform focusing on a near object.
Japanese Patent Laid-open No. Hei 8-43734 (hereinafter, Patent Document 2) proposes a zoom lens including a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, a fourth lens group having positive refractive power, and a fifth lens group having negative refractive power in that order from the object side. In this zoom lens, in magnification variation from the wide angle end to the telephoto end, at least the first lens group and the fifth lens group move toward the object side in such a manner that the air gap between the first lens group and the second lens group increases, and the air gap between the third lens group and the fourth lens group increases, and the air gap between the fourth lens group and the fifth lens group decreases. Furthermore, in magnification variation from the wide angle end to the telephoto end, the third lens group and the fifth lens group integrally move.
The zoom lenses described in the above-mentioned Patent Documents 1 and 2 are ones including five movable groups and facilitate achievement of high magnification and performance enhancement. Furthermore, they are configurations that easily allow size reduction of the lens barrel in the optical axis direction because the amount of movement of the respective groups can be suppressed by dispersing the magnification variation action to the respective groups. Moreover, a positive lens group is employed as the first lens group closest to the object side and a negative lens group is employed as the fifth lens group closest to the image side, and the telephoto ratio of the whole system is decreased. Thereby, the total length of the optical system is shortened.
Furthermore, the above-mentioned Patent Document 1 proposes that the fourth lens group close to the aperture stop is used as a focus group to thereby suppress the lens diameter and weight of the focus group and facilitate size reduction of the drive system thereof and hence the lens barrel. In addition, the focusing sensitivity suitable for moving image photographing can be set by properly setting the lateral magnification of the fourth lens group and the fifth lens group in the range from the wide angle end to the telephoto end.
However, in an embodiment example described in the above-mentioned Patent Document 1, the fourth lens group serving as the focus group is composed of at least two lenses. The individual lens elements of the optical system used in the lens-interchangeable digital camera generally have larger size and heavier weight compared with lens elements used for compact cameras and video camcorders. This applies also to the focus group. If the weight of the focus group is heavy, large driving force is necessary and therefore the size of the actuator becomes larger, which is disadvantageous in size reduction of the lens barrel. Furthermore, the silence in driving is lost and the lens becomes one that is unsuitable for moving image photographing. Therefore, further reduction in the weight of the focus group is required in the optical system of the interchangeable lens used in the lens-interchangeable digital camera.
Furthermore, although the zoom lenses described in the above-mentioned Patent Documents 1 and 2 have five lens groups and are configurations that are advantageous in achievement of high magnification and performance enhancement, the burden of magnification variation is biased toward the fifth lens group and the amount of movement of the fifth lens group in magnification variation is very large. As a result, the amount of movement of the respective groups is large in magnification variation from the wide angle end state to the telephoto end state and it is difficult to shorten the total length of the whole system at the telephoto end. In addition, generally a multistage movement structure is necessary to realize such an optical system and therefore the size increases in the radial direction.
There is a need for the present disclosure to provide a zoom lens and an imaging device that have a small-size, light-weight focus group suitable for moving image photographing.